Governor Andrew Cuomo even offered to back Sen. Roy McDonald if he chose to continue as a third-party candidate, and a poll released by marriage equality advocates indicated the effort could be successful.

New York State Senator Roy McDonald will not try making a third-party run after losing his Republican primary, at least in part because of his vote for marriage equality in the state.

The two-term incumbent announced the decision today after losing his primary by about 100 votes, or less than 1% of the vote, in the low turnout contest on September 13. Mark Grisanti and Stephen Saland, two other Republican senators who voted for the marriage equality legislation last year and ran for re-election, won their primaries, with Saland emerging victorious from a close race this week.

“Roy McDonald stood on the right side of history and voted with the best interest of all New York families at heart when he stood up to support marriage equality," said HRC President Chad Griffin.

After it appeared that McDonald had lost, New York Governor Andrew Cuomo had already made an unprecedented cross-party endorsement Wednesday, sending a two-page letter to the senator that said it “would be my honor to stand with you” should the Republican lawmaker proceed as a third-party candidate after being narrowly defeated in a primary where his marriage equality vote played a role. But he opted against a chance to run on the Independence Party line. A new poll showed McDonald would win a three-way race this November.

McDonald appeared to lose to Saratoga County Clerk Kathy Marchione following a count of paper ballots this week. Marriage equality advocates and opponents had viewed the outcome as critical to their ongoing campaigns in other state legislatures, with donors including New York City Mayor Michael Bloomberg and billionaire hedge fund manager Paul Singer giving large sums of money to the Republican senators.

In the two-page letter first reported by The New York Times, the governor said he believed McDonald’s vote for marriage equality was a “contributing factor” to his defeat, and he accepted “personal responsibility” for his strong advocacy, which included one-on-one lobbying of McDonald and the other senators. He bemoaned the outsized influence of “extremists” from both parties that he said paralyze the political process, which in his view works best when “we elect people of substance and integrity to exercise their best judgment.” The Democratic governor is popular with independents and Republicans in New York.

“I believe one of the greatest challenges facing the integrity of the political system is the disproportionate power of the political extremists – on both sides of the aisle,” he wrote. “I believe government inaction on many levels, and in particular legislative gridlock, is often attributable to fear of political reprisal from those extremists.”

Cuomo told McDonald that while he did not “presume to understand your politics,” he did believe that, “You should not be left to stand alone now.”

Log Cabin Republicans in the state lamented the loss of McDonald but praised his legacy.

"History will remember you as a hero,” said Gregory T. Angelo, state chairman of the Log Cabin Republicans of New York. “While we are disappointed that Senator McDonald has decided not to actively campaign this fall — especially in light of recent polling that suggested he would win in the general election — we understand that this was not an easy decision for the Senator, nor was it one that was reached without a great deal of thought and deliberation."

One of the chief factors McDonald had to consider was whether his candidacy would split the Republican vote with Marchione and allow Claverack town supervisor Robin Andrews, the Democratic nominee, to win. The Republican senate leadership, which has not yet expressed a preference in the general election, is seeking to maintain and grow its slim majority in the chamber, and the conventional wisdom is that a third-party bid from McDonald would have complicated those efforts.

A new poll released Thursday morning by New York Unity PAC, a group funded by Singer and other Republican-affiliated figures including Ken Mehlman, indicates that McDonald would have prevailed over the other two candidates if he had run on the Independence Party line in the Republican-leaning district. The PAC has spent more than $200,000 to support the Republican senators who voted for the marriage equality legislation. Leading LGBT advocacy groups including the Human Rights Campaign, Empire State Pride Agenda and Gill Action Fund, which were heavily involved in the campaign to pass the legislation, had expressed their intentions to stand by McDonald.